This invention relates to low-expansion ceramics having a small coefficient of thermal expansion, an excellent thermal shock-resistance, and an excellent heat-resistance.
With the progress of technology in recent years, demand for material having an excellent thermal shock-resistance and an excellent heat-resistance is increasing. The thermal shock-resistance of ceramics depends on characteristics of the materials, such as the coefficient of thermal expansion, the heat conductivity, the mechanical strength, the Young's modulus, and the Poisson's ratio. The thermal shock-resistance is also affected by the size and shape of the goods concerned and the conditions of heating and cooling or the rate of heat propagation. Among those factors affecting the thermal shock-resistance, the contribution of the coefficient of thermal expansion is especially large, and when the rate of heat propagation is high, the thermal shock-resistance is ruled almost solely by the coefficient of thermal expansion, as well known to those skilled in the art. Accordingly, there is a strong demand for development of low-expansion material with excellent resistance against thermal shock.
As ceramics with a comparatively low thermal expansion, which has a coefficient of thermal expansion in the order of 5 to 20.times.10.sup.-7 (1/.degree. C.) in a temperature range of 25.degree. C. to 800.degree. C., cordierite (MAS) and lithium-aluminum-silicate (LAS) are known. However, such known ceramics has a low melting point, e.g., the melting point of cordierite is 1,450.degree. C. and that of lithium-aluminum-silicate is 1,423.degree. C. For instance, when the ceramics is used to make ceramic honeycombs for catalyst substrate of catalytic purifying apparatus of automobiles, even the honeycomb substrate using cordierite with a high melting point has been found vulnerable to plugging due to melting if the temperature of the catalyst bed is increased by 100.degree. C. to 200.degree. C. over that of conventional catalyst beds. The increase of the temperature of the catalyst bed is caused by modification of the mounting position of the catalytic converter from the conventional location of under bed to engine proximity for improving the purifying efficiency of the catalyst and by design modification involving the mounting of a turbo-charger for improving the fuel economy and the engine output, which modifications cause an increase in the exhaust gas temperature as compared with that of conventional apparatus. Accordingly, the development of low-expansion material having an excellent heat-resistance, which also has an excellent thermal shock-resistance equivalent to or better than that of cordierite, has been strongly demanded.